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Tensin test report

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Tensin test report

1. 1. Tension Test Tensile test Objective To perform the tensile test on the given samples and to determine the associated properties of specimens using universal testing machine. Apparatus  Universal testing machine  Given specimens (cast iron and aluminium)  Vanier calliper  Extensometer Introduction Mechanical testing plays an important role in evaluating fundamental properties of engineering materials as well as in developing new materials and in controlling the quality of materials for use in design and construction. If a material is to be used as part of an engineering structure that will be subjected to a load, it is important to know that the material is strong enough and rigid enough to withstand the loads that it will experience in service. As a result engineers have developed a number of experimental techniques for mechanical testing of engineering materials subjected to tension, compression, bending or torsion loading. Tensile properties indicate how the material will react to forces being applied in tension. A tensile test is a fundamental mechanical test where a carefully prepared specimen is loaded in a very controlled manner while measuring the applied load and the elongation of the specimen over some distance. Tensile tests are used to determine the modulus of elasticity, elastic limit, elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. The most common type of test used to measure the mechanical properties of a material is the Tension Test. The main product of a tensile test is a load versus elongation curve which is then converted into a stress versus strain curve. Since both the engineering stress and the engineering strain are obtained by dividing the load and elongation by constant values (specimen geometry information), the load-elongation curve will have the same shape as the engineering stressstrain curve. The stress-strain curve relates the applied stress to the resulting strain and each material has its own unique stress-strain curve. A typical engineering stress-strain curve is 1|Page
2. 2. Tension Test shown below. If the true stress, based on the actual cross-sectional area of the specimen, is used, it is found that the stress-strain curve increases continuously up to fracture. Elastic Region Stress = s = P/A (Load/Initial cross-sectional area) Strain = e = ∆L/L (Elongation/Initial gage length) Engineering stress and strain are independent of the geometry of the specimen. In start stress and strain are in linear relationship. This is the linear-elastic portion of the curve and it indicates that no plastic deformation has occurred. In this region of the curve, when the stress is reduced, the material will return to its original shape. In this linear region, the line obeys the relationship defined as Hooke's Law where the ratio of stress to strain is a constant. σ = Ee Where σ = engineering stress e = engineering strain E = elastic modulus or young’s modulus The slope of the line in this region where stress is proportional to strain and is called the “modulus of elasticity” or “Young's modulus”. The modulus of elasticity (E) defines the properties of a material as it undergoes stress, deforms, and then returns to its original shape after the stress is removed. It is a measure of the stiffness of a given material Plastic region The part of the stress-strain diagram after the yielding point. At the yielding point, the plastic deformation starts. Plastic deformation is permanent. At the maximum point of the stress-strain diagram (σ UTS), necking starts. Yield Point In ductile materials, at some point, the stress-strain curve deviates from the straight-line relationship and Law no longer applies as the strain increases faster than the stress. From this point on in the tensile test, some permanent deformation occurs in the specimen and the material is said to react plastically to any further increase in load or stress. The material will not return to its original, unstressed condition when the load is removed. In brittle materials, 2|Page
3. 3. Tension Test little or no plastic deformation occurs and the material fractures near the end of the linearelastic portion of the curve. For most engineering design and specification applications, the yield strength is used. The yield strength is defined as the stress required to produce a small, amount of plastic deformation. The offset yield strength is the stress corresponding to the intersection of the stress-strain curve and a line parallel to the elastic part of the curve offset by a specified strain (in the US the offset is typically 0.2% for metals and 2% for plastics while in UK offset method is 0.1% or 0.5%. Stress corresponding to 0.1% strain is known as proof strength). In some materials there is upper yield point and lower yield point. In these materials load at yield point suddenly drops this is known as yield point. After decreasing load, strain increases while load remain almost constant. This phenomena is known as yield-point elongation. After yielding stress increases. The deformation occurring throughout the yield-point elongation is heterogeneous. At the upper yield point, a discrete band of deformed metal, often readily visible, appears at a stress concentration, such as a fillet. Coincident with the formation of the band, the load drops to the lower yield point. The band then propagates along the length of the specimen, causing the yield-point elongation. Figure 1: yield elongation phenomena in material by volume 8 - Mechanical Testing and Evaluation ASM hand book Figure 2: Upper and lower yield point in material by ASM hand book volume8 - Mechanical Testing And Evaluation A similar behaviour occurs with some polymers and superplastic metal alloys, where a neck forms but grows in a stable manner, with material being fed into the necked region from the thicker adjacent regions. This type of deformation in polymers is called “drawing”. 3|Page
4. 4. Tension Test Ultimate Tensile Strength The ultimate tensile strength (UTS) or, more simply, the tensile strength, is the maximum engineering stress level reached in a tension test. The strength of a material is its ability to withstand external forces without breaking. In brittle materials, the UTS will at the end of the linear-elastic portion of the stress-strain curve or close to the elastic limit. In ductile materials, the UTS will be well outside of the elastic portion into the plastic portion of the stress-strain curve. Measures of Ductility The ductility of a material is a measure of the extent to which a material will deform before fracture. The amount of ductility is an important factor when considering forming operations such as rolling and extrusion. It also provides an indication of how visible overload damage to a component might become before the component fractures. In general, measurements of ductility are of interest in three ways: 1. To indicate the extent to which a metal can be deformed without fracture in metalworking operations such as rolling and extrusion. 2. To indicate to the designer, in a general way, the ability of the metal to flow plastically before fracture. 3. To serve as an indicator of changes in impurity level or processing conditions. Ductility measurements may be specified to assess material quality even though no direct relationship exists between the ductility measurement and performance in service. The conventional measures of ductility are the engineering strain at fracture (usually called the elongation) and the reduction of area at fracture. Both of these properties are obtained by fitting the specimen back together after fracture and measuring the change in length and crosssectional area.  % elongation = Lf−Lo 𝐿𝑜 ˣ 100 Where Lf = final length Lo = initial length 4|Page
5. 5. Tension Test  % Reduction in area = 𝐴𝑜−𝐴𝑓 𝐴𝑜 ˣ 100 Where Ao = initial length Af = final length Resilience Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. The associated property is the modulus of resilience, Ur which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding Ɛ𝑦 Ur = ∫ σdx 0 Assuming a linear elastic region, Ur = ½ σyƐy Toughness Toughness is a mechanical term that may be used in several contexts. For one, toughness (or more specifically, fracture toughness) is a property that is indicative of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present. Because it is nearly impossible (as well as costly) to manufacture materials with zero defects (or to prevent damage during service), fracture toughness is a major consideration for all structural materials. Another way of defining toughness is as the ability of a material to absorb energy and plastically deform before fracturing. For dynamic (high strain rate) loading conditions and when a notch (or point of stress concentration) is present, notch toughness is assessed by using an impact test. Several mathematical approximations for the area under the stress-strain curve have been suggested. For ductile metals that have a stress-strain curve like that of the structural steel, the area under the curve can be approximated by 5|Page
6. 6. Tension Test For brittle materials, the stress-strain curve is sometimes assumed to be a parabola, and the area under the curve is given by Poisson's ratio Poisson’s ratio is defined as the negative of the ratio of the lateral strain to the axial strain for a uniaxial stress state. Only two of the elastic constants are independent so if two constants are known, the third can be calculated using the following formula: E = 2G (1 + v) Where: E = modulus of elasticity (Young's modulus) V = Poisson's ratio G = modulus of rigidity (shear modulus) Necking Up to maximum stress deformation is homogeneous and material deform plastically. But after maximum stress delocalized deformation takes place. After UTS stresses are concentrated at weaker portion of the specimen and a neck is formed at that there. Load bearing capacity of material decrease due to necking. Up to the point at which the maximum force occurs, the strain is uniform along the gage length; that is, the strain is independent of the gage length. Figure3: necking area in sample by AMS metal handbook volume 8 - Mechanical Testing and Evaluation However, once necking begins, the gage length becomes very important. Specimen for Tension Test In standard tensile specimen normally, the cross section is circular, but rectangular specimens are also used. The “dogbone” specimen configiration was chosen so that during testing, Figure4: standard specimen shape by AMS metal handbook volume 8 - Mechanical Testing and Evaluation 6|Page
7. 7. Tension Test deformation is confined to the narrow center region (which has uniform cross section along its length) and also to reduce the likelihood of fracture at the end of the specimen. The standard diameter is approximately 12.8 mm (0.5 in.), whereas the reduced section length should be at least four times this diameter; 60 mm is common. Gauge length is used in ductility computations, as discussed in Section Figure5: dimension of standard specimen Materials Science and Engineering by D. Callister 6.6; the standard value is 50 mm (2.0 in.) Machine for Tension Test According to the loading type, there are two kinds of tensile testing machines; 1) Screw Driven Testing Machine: During the experiment, elongation rate is kept constant. 2) Hydraulic Testing Machine: Keeps the loading rate constant. The loading rate can be set depending on the desired time to fracture. A tensile load is applied to the specimen until it fractures. During the test, the load required to make a certain elongation on the material is recorded. Procedure  Put gage marks on the specimen  Measure the initial gage length and diameter  Select a load scale to deform and fracture the specimen. Note that that tensile strength of the material type used has to be known approximately.  Record the maximum load  Conduct the test until fracture.  Measure the final gage length and diameter. The diameter should be measured from the neck Figure 6: specimen and machine arrangement for tension test by AMS metal handbook volume 8 - Mechanical Testing and Evaluation 7|Page
8. 8. Tension Test Calculation For Aluminium specimen  Initial length of specimen = Lo = 100mm  Initial diameter = Do=  Final length of specimen = Lf =  Final area of specimen = Af =  Yield strength = 85.59 kN  Ultimate tensile strength = 108.8731628  Fracture strength = 137.755N  Modulus of resilience =  Modulus of toughness = For Cast Iron specimen  Initial length of specimen = Lo = 100mm  Initial diameter = Do=  Final length of specimen = Lf = 68.5mm  Final area of specimen = Df =  Yield strength =  Ultimate tensile strength = 748.502994 kN  Fracture strength = 646.43 kN  Modulus of resilience =  Modulus of toughness = Application of Tension Test Tensile testing is used to guarantee the quality of components, materials and finished products within a wide range industries. Typical applications of tensile testing are highlighted in the following sections on:  Aerospace Industry  Automotive Industry  Beverage Industry  Construction Industry  Electrical and Electronics Industry 8|Page
9. 9. Tension Test  Medical Device Industry  Packaging Industry  Paper and Board Industry  Pharmaceuticals Industry  Plastics, Rubber and Elastomers Industry  Safety, Health, Fitness and Leisure Industry  Textiles Industry References  www.azom.com  Materials Science and Engineering by D. Callister  8 - Mechanical Testing and Evaluation  Mechanical metallurgy by Dieter 9|Page